This invention relates to expandable medical implants for maintaining support of a body lumen.
An important use of stents is found in situations where part of the vessel wall or stenotic plaque blocks or occludes blood flow in the vessel. Often, a balloon catheter is utilized in a percutaneous transluminal coronary angioplasty procedure to enlarge the occluded portion of the vessel. However, the dilation of the occlusion can cause fissuring of atherosclerotic plaque and damage to the endothelium and underlying smooth muscle cell layer, potentially leading to immediate problems from flap formation or perforations in the vessel wall, as well as long-term problems with restenosis of the dilated vessel. Implantation of stents can provide support for such problems and prevent re-closure of the vessel or provide patch repair for a perforated vessel. Further, the stent may overcome the tendency of diseased vessel walls to collapse, thereby maintaining a more normal flow of blood through that vessel.
Examples of prior developed stents have been described by Balcon et al., "Recommendations on Stent Manufacture, Implantation and Utilization," European Heart Journal (1997), vol. 18, pages 1536-1547, and Phillips, et al., "The Stenter's Notebook," Physician's Press (1998), Birmingham, Mich. The first stent used clinically was the self-expanding "Wallstent" which comprised a metallic mesh in the form of a Chinese fingercuff. These stents were cut from elongated tubes of wire braid and, accordingly, had the disadvantage that metal prongs from the cutting process remained at the longitudinal ends thereof. The inherent rigidity of the cobalt based alloy with a platinum core used to form the stent together with these terminal prongs made navigation of the blood vessels to the locus of the lesion difficult as well as risky from the standpoint of injury to healthy tissue along the passage to the target vessel. Furthermore, once placed, the continuous stresses from blood flow and cardiac muscle activity created significant risks of thrombosis and damage to the vessel walls adjacent to the lesion, leading to restenosis. A major disadvantage of these types of stents were that their radial expansion was associated with significant shortening in their length, resulting in unpredictable longitudinal coverage when fully deployed.
Among subsequent designs, some of the most popular have been the Palmaz-Schatz slotted tube stents. Originally, the Palmaz-Schatz stents consisted of slotted stainless steel tubes comprising separate segments connected with articulations. Later designs incorporated spiral articulation for improved flexibility. These stents are delivered to the affected area by means of a balloon catheter, and are then expanded to the proper size. The Palmaz-Schatz designs exhibit moderate longitudinal shortening upon expansion, with some decrease in diameter, or recoil, after deployment. Furthermore, the expanded metal mesh is associated with relatively jagged terminal prongs, which increase the risk of thrombosis and/or restenosis.
Another type of stent involves a tube formed of a single strand of tantalum wire, wound in a sinusoidal helix; these are known as the Wiktor stents. They exhibit increased flexibility compared to the Palmaz-Schatz stents; however, they do not provide sufficient scaffolding support for many applications, including calcified or bulky vascular lesions. Further, the Wiktor stents also exhibit some recoil after radial expansion.
Another form of metal stent is a heat expandable device using Nitinol or a tin-coated, heat expandable coil. This type of stent is delivered to the affected area on a catheter capable of receiving heated fluids. Once properly situated, heated saline is passed through the portion of the catheter on which the stent is located, causing the stent to expand. Numerous difficulties have been encountered with this device, including difficulty in obtaining reliable expansion, and difficulties in maintaining the stent in its expanded state.
Self-expanding stents are problematic in that exact sizing, within 0.1 to 0.2 mm expanded diameter, is necessary to adequately reduce restenosis. However, self-expanding stents are currently available only in 0.5 mm increments. Thus, greater flexibility in expanded size is needed.
Stents can be deployed in a body lumen by means appropriate to their design. One such method would be to fit the collapsed stent over an inflatable element of a balloon catheter and expand the balloon to force the stent into contact with the body lumen. As the balloon is inflated, the problem material in the vessel is compressed in a direction generally perpendicular to the wall of the vessel which, consequently, dilates the vessel to facilitate blood flow therethrough. Radial expansion of the coronary artery occurs in several different dimensions and is related to the nature of the plaque. Soft, fatty plaque deposits are flattened by the balloon and hardened deposits are cracked and split to enlarge the lumen. It is desirable to have the stent radially expand in a uniform manner.
Alternatively, the stent may be mounted onto a catheter which holds the stent as it is delivered through the body lumen and then releases the stent and allows it to self-expand into contact with the body lumen. This deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the catheter.
In summary, significant difficulties have been encountered with all prior art stents. Each has its percentage of thrombosis, restenosis and tissue in-growth, as well as various design-specific disadvantages. Thus, there is a need for an improved stent: one that has relatively smooth marginal edges, to minimize restenosis; one that is small enough and flexible enough when collapsed to permit delivery to the affected area; one that is sufficiently flexible upon deployment to conform to the shape of the affected body lumen; one that expands uniformly to a desired diameter, without change in length; one that maintains the expanded size, without significant recoil; and one that has sufficient scaffolding to provide a clear through-lumen.